(1) Field of the Invention
This invention relates to a continuous process for preparing a fiber reinforced plastic (hereinafter referred to as FRP) flat plate by means of curing with an ionizing radiation, preferably an electron beam. Particularly, this invention comprises irradiating with an ionizing radiation an FRP plate molding material both top and back surfaces of which are covered with a film-like sheeting, characterized in that the molding material covered with the sheetings is continuously transported and tensions are given to the covered sheetings in both lengthwise and widthwise directions in the course of processing.
(2) Description of the Prior Art
Generally, FRP plates have been prepared by impregnating a thermosetting resin such as unsaturated polyester resin containing a curing catalyst in glass fibers, and curing preferably by means of a hot press method.
The heat-curing process has also been applied to a continuous molding of an FRP flat plate. However, the continuous heat-curing process has serious defects in large curing strain such as warps and cracks in appearance and formation of bubbles owing to violent exothermic reaction upon curing. When the generation of heat of reaction is regulated to lower, curing takes several to several-ten minutes. Therefore, it is difficult to continuously prepare an FRP flat plate more than 3 mm in thickness even by employing a large curing furnace. Furthermore, since such curing process takes a considerable long period of time and a liquid resin impregnated in reinforcing fiber material is of fluid nature, it is difficult to keep an FRP molding material in uniform shape and thickness until the impregnated resin is sufficiently cured. Therefore, the resulting product is inferior in mechanical properties to hot-pressed product.
It is known that FRP molding material in small size such as 24 .times. 12 inches which had been pressed in discontinuously transported on a conveyer and irradiated with electron beam to form FRP plate, as reported in Society of plastics Engineerings Journal, April 1967, pages 33 - 73. However, this process remains in an experimental scale and is not commercially available, since the warps, creases and unevenness take place more or less on the cured FRP plate. Such defects in the course of curing are further enhanced when a continuous FRP molding material is cured continuously by means of an ionizing radiation such as electron beam, since there was no useful means to maintain the FRP molding material flat and uniform in the course of irradiation. Therefore, it is clear that the continuous process by means of heat curing can not be changed to a continuous process by means of a radiation curing, especially an electron beam curing, by simply replacing heating with irradiation.
It is easily understood by those skilled in the art that a simple replacement of heating with irradiation in the continuous process by means of heat curing could not create the present invention in which a continuous FRP flat plate without strains is advantageously prepared by means of an ionizing radiation.
In summary, the commercially feasible FRP flat plate could have been prepared only by way of a hot press method. However, the hot press method requires a discontinuous processing operation, a long period of time for curing and cooling and a larger scale of the apparatus. Further, an FRP plate of continuous long size can not be obtained by the hot press method.
Incidentally, the conventional FRP flat plates having glossy surfaces have been commercially prepared with a hot press equipped with pressing plates having smooth surfaces such as metal-plated surfaces. Owing to the characteristics of the hot press, many problems in preparing an FRP plate are substantially eliminated, such as non-uniformity of resin materials, formation of bubbles, warps of the plate, rising of fiber materials caused by elastic recovery thereof, sink caused by curing volume contraction, unevenness of the surfaces of the like. This invention advantageously eliminates the above-mentioned problems without employing the conventional hot press and provides a continuous FRP flat plate which could not be prepared by the hot press method.